In stock Zen Cart, the sidebars are #navColumnOne and #navColumnTwo, with inner divs #navColumnOneWrapper and #navColumnTwoWrapper; the center is .centerColumn. Some commercial templates drastically change the element names, so I can't say for sure what will work.

The best thing is to get Firefox and its Web Developer and Firebug extensions. These can tell you what tags apply to each page element, and the styling that controls it. You can edit the CSS and see the result instantly, so you know what permanent changes you want to make.